The present invention relates to tape storage systems, and more specifically, to hybrid servo pattern configurations for use with magnetic tape recording systems and products.
Timing-based servo (TBS) is a technology which was developed for linear tape drives in the late 1990s. In TBS systems, recorded servo patterns include transitions with two different azimuthal slopes, thereby forming a chevron-type pattern. These patterned transitions allow for an estimate of the head lateral position to be determined by evaluating the relative timing of pulses generated by a servo reader reading the patterns as they are passed over the servo reader.
In a TBS format, the servo pattern is prerecorded in several bands distributed across the tape. Typically, five or nine servo pattern bands are included on a given tape which runs about parallel to a longitudinal axis of the tape. Data is recorded in the regions of tape located between pairs of the servo bands. In read/write heads of IBM linear tape-open (LTO) and Enterprise tape drives, two servo readers are normally available per head module, from which longitudinal position (LPOS) information as well as a position error signal (PES) may be derived. Effective detection of the TBS patterns is achieved by a synchronous servo channel employing a matched-filter interpolator/correlator, which ensures desirable filtering of the servo reader signal.
Although TBS patterns have historically been able to provide sufficient positioning data while reading from and/or writing to magnetic tape, conventional products have begun to experience setbacks in performance efficiency. Specifically, as track densities continue to increase for tape media and tape drives, accurately controlling the lateral position of a magnetic head and/or skew of the magnetic head with respect to tape by using feedback generated by reading the TBS patterns has become increasingly difficult. In fact, conventional servo based implementations may not be sufficiently accurate to ensure adequate positioning of the data readers and writers that move along data tracks of magnetic tape having a sufficiently high track density. Furthermore, the repetition rate of the head lateral position estimates may be too low to ensure proper track-following operation, as tape velocity varies during use. The repetition rate of the head lateral position estimates may additionally be unable to support future actuators with larger bandwidths. It is also important to monitor tape dimensional stability (TDS), particularly as track density and tape capacity continue to increase.
However, in the past tape skew and TDS measurements have been determined from the information from servo bands on both sides of a head module, or information from servo readers on multiple head modules. In other words, to compute skew and/or TDS, conventional products have needed to obtain valid servo information from more than one servo band and/or more than one head module. This makes such conventional head modules particularly susceptible to degraded performance and/or being rendered completely useless by servo defects, scratches caused by asperities on the surface of the magnetic tape, etc.
Some products have implemented servo bands having a hybrid servo pattern in an attempt to alleviate some of the foregoing shortcomings. Hybrid servo patterns employ a high density (HD) servo pattern in addition to the TBS pattern, thereby providing some additional information. However, products implementing hybrid servo patterns have been unable to achieve functionality while also enabling backward compatibility in a single tape drive. Backward compatibility is highly desirable for removable storage media such as magnetic tape. For instance, backward compatibility allows a given tape drive to support multiple different generations of magnetic tape. Accordingly, backward compatibility allows users to maximize flexibility of tape media resource arrangements available to them.
To achieve backward compatibility among multiple generations of magnetic tape, it is desirable that a number of data bands relative to servo bands maintain a standard ratio while the data capacity of magnetic tapes increase. Moreover, it is desirable that servo readers on a single head module are compatible with various different servo band formats. However, this has served as a significant issue for conventional products thus far. Accordingly, achieving a magnetic tape and/or system which is able to continue to increase data capacity, while also improving data track following performance, as well as maintaining a standard ratio of data bands relative to servo bands is greatly desired.